NASA's Mars Exploration Rover Opportunity spent its seventh anniversary of its landing on Mars investigating a crater called "Santa Maria," which has a diameter of about the length of a football field.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

This is the longest living autonomous robotic system that has functioned without physical human intervention. It represents a benchmark in both robotics history, as well as in in-situ planetary exploration.

This image captured by the Mars Exploration Rover Opportunity's rear hazard-identification camera shows the now-empty lander that carried the rover 283 million miles to Meridiani Planum, Mars.

This image from NASA's Mars Exploration Rover Spirit shows the rover's robotic arm extended to the rock called Adirondack. The Moessbauer spectrometer, an instrument for identifying the minerals in rocks and soils, is placed against the rock.

The various algorithms that were uploaded during the mission extensions have enabled a number of scientific discoveries that would not have been possible otherwise.

NASA's Mars Exploration Rover Spirit spent 2006's Martin Luther King, Jr. holiday weekend analyzing a remarkable exposure of bright, loose material. Spirit discovered the material while driving toward "Home Plate" along the floor of the basin south of "Husband Hill" in Gusev Crater. These images from Spirit's panoramic camera (Pancam) show some of the most colorful deposits photographed on the surface of Mars.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

Sojourner: First instance of a mobile robot that functioned in an unknown environment with no physical human interaction.

An enhanced version of the famous Mars Pathfinder photo of the lander and Sojourner rover. This browse version shows only part of the 360-degree panorama. Click on the above to access the high-resolution version, which includes the whole image.

Sojourner showcased the ability for planetary robots to function with some intelligence in enabling planetary science objectives. It proved that robots were now capable of roving on other planets, thus opening up amazing possibilities and pushing the boundaries for future scientific discoveries.

Before moving on to explore more of Mars, NASA's Mars Exploration Rover Spirit looked back at the long and winding trail of twin wheel tracks that the rover created to get to the top of "Husband Hill."

Mars Exploration Rovers: Spirit and Opportunity are now considered the gold-standard of robotics for long-term exploration in unknown and hazardous environments. For years-to-come, scientists will still be analyzing and evaluating the science data collected by the two-rover system sent to increase our understanding of the Martian planet.

The rover's navigation camera captured this view back into the crater just after finishing a 6.8-m (22-foot) drive that brought Opportunity out onto level ground during the mission's 1,634th Martian day, or sol (28 Aug. 2008).

Another great achievement is humanoids interacting with astronauts. Although they're still running through functional capabilities, it's still a significant achievement.